1. Field of the Invention
This invention relates to a process for treating air-borne dusts containing silicon dioxide in order to convert the silicon dioxide into a silicic acid or a silicate. The air-borne dust is suitably a waste product from a silicon metal or silicon alloy production process and usually harmful to the environment.
2. Prior Art
Several processes are known for the production of alkali silicates, from the aqueous solutions of which (water glass) finely dispersed silicic acids and silicates are obtained by means of acid silica gel or by special precipitation methods.
One of these processes is known as the dry process, which is practiced generally today. In it, silicon dioxide (quartz) is caused to react with soda in a melt. In another process known as the wet process, quartz-sand is made to react under pressure with an aqueous alkali-hydroxide solution.
The dry process has the disadvantage that for carrying out the reaction in the molten state, a great expenditure of heat energy is required. In addition, the melting process requires very expensive apparatus, and furnaces and melting tanks are exposed to great wear because of the high operating temperatures required. It is characterized in addition by a particularly careful selection of the silicon dioxide material, especially with a view toward the content of iron and aluminum oxide.
In the case of the wet method, the conversion is difficult to carry out quantitatively, because when natural sand is used, it reacts only incompletely and relatively slowly. According to this wet processing, one can only operate discontinuously and thus work intensively. The resulting water glass is dilute and has a low modulus (SiO.sub.2 :Na.sub.2 O), which means an increased use of alkali. In addition, the yield is poor since the conversion does not take place quantitatively.
In order to avoid some of these disadvantages, the relatively easily reacting, naturally occurring volcanic sands are used increasingly as starting silicic acid material.
For the production of particularly pure alkali silicates, especially for scientific purposes, it has been proposed to use pyrogenically produced, chemically pure, amorphous silicic acid as a starting product. This use, however, is disadvantageous because the alkali silicate produced may only be used for special purposes due to the high cost of the starting material.
Therefore, for the production of water glass from as cheap as possible raw materials, workers in the art have used waste silicic acids, such as those obtained from the AlF.sub.3 -process from H.sub.2 SiF.sub.6 and Al(OH).sub.3 : in the case of acid decomposition of raw phosphates for the production of phosphoric acid, phosphates and fertilizers, there develops as a by-and waste product volatile silicon tetrafluoride.
Precipitated in water, hexafluoro silicic acid is formed from this, which may be processed further to valuable fluoro compounds. In case of such processes for the production of Na.sub.3 AlF.sub.6, AlF.sub.3, NaF and HF basically silicic acid is obtained as a by-product. The separated silicic acid may be reprocessed according to a known process (Germ. Pat. No. 2,219,503) to water glass with limited stability and with a high modulus, but without complete removal of the fluoride present. The precipitated silicic acids, which may be produced from this, contain corresponding quantities of fluoride and may thus only have a limited use. Thus, for example, they may not be used for pharmaceutical preparations and as fodder and/or food additives. After it has become possible to produce water glass from SiO.sub.2 -containing waste products stemming from the decomposition of raw phosphates also other waste products containing high amounts of SiO.sub.2 have been processed into water glass according to known methods.
A large quantity of volatile gases, which initially contain silicon mon-oxide, develop during the production of silicon metal, silicon alloys, silicides and Si-carbide, which are produced in an electro furnace by metallurgical processes. These gases, upon access to atmospheric oxygen, oxidize into a highly dispersed silicon dioxide. Additionally, the reaction gases also contain solid impurities, which together with the SiO.sub.2 are separated in considerable quantities as air-borne dust from the exhaust gases. The resulting fine powder has a high content of amorphous SiO.sub.2. At the same time, the impurities contained in the starting mixture, such as sulfur and phosphorus, are frequently present in the air-borne dust. The carbon used as a reducing agent in the form of, for example, graphite electrodes, also in part gets into the waste gas in a finely distributed form and gives the air-borne dust a blackish gray color. Furthermore, the separation products contain impurities originating from the reaction of quartz with, for example, petroleum coke, as well as among other things organic substances liberated by thermal decomposition from the bituminous coal tar or dextrines used as a binder in the electrode material, which reach the SiO.sub.2 -containing exhaust gas as cracking products. During the separation of the air-borne dust from the waste gas, these impurities are firmly absorbed by very finely distributed silicic acid.
This air-borne dust waste is obtained in considerable quantities: for instance, in case of the production of 1 ton of silicon-iron alloy by reduction, 0.2-0.5 tons of dust will develop. The reuse of this dust as a substitute for quartz has, to be sure, been attempted, but it has not been very profitable because of the necessity of a granulating process. Since, hiterto, no significant technically and economically practical possibility of use has resulted, the air-borne dust waste is either blown in the form of smoke into the atmosphere and constitutes a main cause for air pollution or it is collected and introduced into the sea or into rivers or moved to a dump, depending on the location of the factory. These measures still result in annoyances because of the high degree of fine distribution of the dust.
To be sure, the reprocessing of fine powdery, amorphous silicic acid by decomposition with alkali hydroxides in the wet process has already been proposed, because silicic acid is obtained in the production of silicon metal or its alloys as an industrial waste in large quantities. At the same time, silica gel is obtained from these finely powdered, amorphous silicic acid containing air-borne dusts by way of water glass by means of acids. The silica gel, because of the previously mentioned impurities, does not correspond to the conditions demanded most, and therefore may only be used to a very limited extent in some areas of application.
Up to this time these air-borne dusts polluting the environment could not be supplied to any kind of sufficient, technical use despite all efforts made (cf. European Chemical News, 10, 1, 1975, p. 15).
Therefore, there exists a need in the art for a process for the reprocessing of silicon dioxide-containing air-borne dust waste contaminating (polluting) the environment, such wastes emanating from silicon metal and silicon alloy production processes. There is a need to convert the silicon dioxide into silicic acids and silicates, which represent high grade products with a wide range of uses, for example, as reinforcing fillers in the rubber and plastics sector, as well as in many other fields, such as in the paint, varnish, lacquer, paper, cosmetic and pharmaceutical industries, as well as in feed and pesticide production.